The present invention relates to methods and apparatus for the production of extruded plastic film. More particularly, the invention relates to methods and apparatus for the production of extruded plastic film having strips of a second thermoplastic material embedded in one or both surfaces of the film.
Several methods and apparatus are presently disclosed in the art for the coextrusion of strips, ribbons and the like of a second material wholly or partly within a surrounding matrix of a first thermoplastic material.
For example, commonly-assigned U.S. Pat. No. 3,565,737 to Lefevre et al. (hereafter, the '737 patent) discloses a composite film which has increased cling, wherein the composite film has a body portion made from a low cling plastic with strips of relatively high cling material embedded in each major surface of the body portion.
An apparatus illustrated in the '737 patent for producing the composite film uses two extruders. One extruder provides a plastified film forming thermoplastic material to a sheeting die to form the main body of the coposite film. A second extruder, meanwhile, feeds the adhesive material in the form of strips into the plastified mass of main body material prior to the two materials being forced out through a die opening. Die arrangements for accomplishing this feeding of the second material into the first include arrangements for discharging the second material both cross-currently and co-currently into the first material.
Commonly-assigned U.S. Pat. No. 3,444,031 to Schrenk discloses a method and apparatus for making a "light diffusing panel", involving the co-current extrusion of a plurality of elongated parallel strips of an optically different thermoplastic composition within a surrounding matrix material.
Significant drawbacks are seen with such known methods and apparatus, however, as regards the production of films from certain combinations of materials.
Using for purposes of illustration the polyethylene/ethylene-vinyl acetate film exemplified in the '737 patent, it is well known that ethylene-vinyl acetate copolymers (EVA) can degrade when extruded above certain temperatures, depending on the residence time of such copolymers in the extrusion apparatus.
In known apparatus which fully or partially encapsulate the EVA copolymers in a polyethylene matrix by having the EVA flow co-currently with the polyethylene in a conduit which terminates inside of a sheeting die, the transfer of heat from the polyethylene across the conduit walls is a potential source of degradation of the EVA.
Cross-current feeding of the EVA into the higher temperature polyethylene sheet prior to the exit of the composite film from the sheeting die would be advantageous, then, from the perspective of limiting heat transfer from the polyethylene to heat transferred from the point of introduction of the EVA to subsequent cooling of the composite film.
Cross-current introduction of lower viscosity materials such as EVA into the polyethylene sheet, however, results in shear migration of the EVA as the composite body of materials formed thereby progresses through the die The process thus results in strips of the EVA which are wider than the stream of EVA injected into the polyethylene sheet.
Where the distance between the point of introduction of the EVA and the exit of the die is not particularly large, as with the apparatus described in the '737 patent, the shear migration may not be especially troublesome.
Where the flexible die lip technology commonly used for commercial width extrusion dies is sought to be employed, however die than the point of introduction of the EVA is a greater distance away from the exit of the die than in the '737 patent It has been observed that while this increased distance may not be so large as to cause appreciably greater degradation of the strip-comprising EVA copolymers, shear migration of the EVA strips in the composite film can be sufficiently pronounced that the strips begin to overlap and cause blocking when the film is wound onto a roll.
It would thus be desirable also if the shear migration associated with cross-current feeding of lower viscosity strip-comprising materials at locations removed from the die lips were able to be limited to an extent, while at the same time allowing for the use of the commonly-practiced flexible die lip technology.